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.TH MQ_RECEIVE 3C "Feb 5, 2008"
.SH NAME
mq_receive, mq_timedreceive, mq_reltimedreceive_np \- receive a message from a
message queue
.SH SYNOPSIS
.LP
.nf
#include <mqueue.h>

\fBssize_t\fR \fBmq_receive\fR(\fBmqd_t\fR \fImqdes\fR, \fBchar *\fR\fImsg_ptr\fR, \fBsize_t\fR \fImsg_len\fR,
     \fBunsigned *\fR\fImsg_prio\fR);
.fi

.LP
.nf
#include <mqueue.h>
#include <time.h>

\fBssize_t\fR \fBmq_timedreceive\fR(\fBmqd_t\fR \fImqdes\fR, \fBchar *restrict\fR \fImsg_ptr\fR,
     \fBsize_t\fR \fImsg_len\fR, \fBunsigned *restrict\fR \fImsg_prio\fR,
     \fBconst struct timespec *restrict\fR \fIabs_timeout\fR);
.fi

.LP
.nf
\fBssize_t\fR \fBmq_reltimedreceive_np\fR(\fBmqd_t\fR \fImqdes\fR,
     \fBchar *restrict\fR \fImsg_ptr\fR, \fBsize_t\fR \fImsg_len\fR,
     \fBunsigned *restrict\fR \fImsg_prio\fR,
     \fBconst struct timespec *restrict\fR \fIrel_timeout\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBmq_receive()\fR function receives the oldest of the highest priority
message(s) from the message queue specified by \fImqdes\fR. If the size of the
buffer in bytes, specified by \fImsg_len\fR, is less than the \fBmq_msgsize\fR
member of the message queue, the function fails and returns an error.
Otherwise, the selected message is removed from the queue and copied to the
buffer pointed to by \fImsg_ptr\fR.
.sp
.LP
If the value of \fImsg_len\fR is greater than {\fBSSIZE_MAX\fR}, the result is
implementation-defined.
.sp
.LP
If \fImsg_prio\fR is not \fINULL\fR, the priority of the selected message is
stored in the location referenced by \fImsg_prio\fR.
.sp
.LP
If the specified message queue is empty and  \fBO_NONBLOCK\fR is not set in the
message queue description associated with \fImqdes\fR, (see \fBmq_open\fR(3C)
and \fBmq_setattr\fR(3C)), \fBmq_receive()\fR blocks, waiting until a message
is enqueued on the message queue, or until \fBmq_receive()\fR is interrupted by
a signal. If more than one process (or thread) is waiting to receive a message
when a message arrives at an empty queue, then the process of highest priority
that has been waiting the longest is selected to receive the message. If the
specified message queue is empty and  \fBO_NONBLOCK\fR is set in the message
queue description associated with \fImqdes\fR, no message is removed from the
queue, and \fBmq_receive()\fR returns an error.
.sp
.LP
The \fBmq_timedreceive()\fR function receives the oldest of the highest
priority messages from the message queue specified by \fImqdes\fR as described
for the \fBmq_receive()\fR function. However, if \fBO_NONBLOCK\fR was not
specified when the message queue was opened with the \fBmq_open\fR(3C)
function, and no message exists on the queue to satisfy the receive, the wait
for such a message is terminated when the specified timeout expires. If
\fBO_NONBLOCK\fR is set, this function is equivalent to \fBmq_receive()\fR.
.sp
.LP
The \fBmq_reltimedreceive_np()\fR function is identical to the
\fBmq_timedreceive()\fR function, except that the timeout is specified as a
relative time interval.
.sp
.LP
For \fBmq_timedreceive()\fR, the timeout expires when the absolute time
specified by \fIabs_timeout\fR passes, as measured by the \fBCLOCK_REALTIME\fR
clock (that is, when the value of that clock equals or exceeds
\fIabs_timeout\fR), or if the absolute time specified by \fIabs_timeout\fR has
already been passed at the time of the call.
.sp
.LP
For \fBmq_reltimedreceive_np()\fR, the timeout expires when the time interval
specified by \fIrel_timeout\fR passes, as measured by the \fBCLOCK_REALTIME\fR
clock, or if the time interval specified by \fIrel_timeout\fR is negative at
the time of the call.
.sp
.LP
The resolution of the timeout is the resolution of the \fBCLOCK_REALTIME\fR
clock. The \fBtimespec\fR argument is defined in the <\fBtime.h\fR> header.
.sp
.LP
Under no circumstance does the operation fail with a timeout if a message can
be removed from the message queue immediately. The validity of the timeout
parameter need not be checked if a message can be removed from the message
queue immediately.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBmq_receive()\fR, \fBmq_timedreceive()\fR, and
\fBmq_reltimedreceive_np()\fR return the length of the selected message in
bytes and the message is removed from the queue. Otherwise, no message is
removed from the queue, the functions return a value of \fB\(mi1\fR, and sets
\fBerrno\fR to indicate the error condition.
.SH ERRORS
.sp
.LP
The \fBmq_receive()\fR, \fBmq_timedreceive()\fR, and
\fBmq_reltimedreceive_np()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEAGAIN\fR \fR
.ad
.RS 13n
\fBO_NONBLOCK\fR was set in the message description associated with
\fImqdes\fR, and the specified message queue is empty.
.RE

.sp
.ne 2
.na
\fB\fBEBADF\fR \fR
.ad
.RS 13n
The \fImqdes\fR argument is not a valid message queue descriptor open for
reading.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR \fR
.ad
.RS 13n
The function was interrupted by a signal.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 13n
The process or thread would have blocked, and the timeout parameter specified a
nanoseconds field value less than zero or greater than or equal to 1,000
million.
.RE

.sp
.ne 2
.na
\fB\fBEMSGSIZE\fR \fR
.ad
.RS 13n
The specified message buffer size, \fImsg_len\fR, is less than the message size
member of the message queue.
.RE

.sp
.ne 2
.na
\fB\fBETIMEDOUT\fR\fR
.ad
.RS 13n
The \fBO_NONBLOCK\fR flag was not set when the message queue was opened, but no
message arrived on the queue before the specified timeout expired.
.RE

.sp
.LP
The \fBmq_receive()\fR, \fBmq_timedreceive()\fR, and
\fBmq_reltimedreceive_np()\fR functions may fail if:
.sp
.ne 2
.na
\fB\fBEBADMSG\fR \fR
.ad
.RS 12n
A data corruption problem with the message has been detected.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
l | l
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
_
Standard	See below.
.TE

.sp
.LP
For \fBmq_receive()\fR and \fBmq_timedreceive()\fR. see \fBstandards\fR(7).
.SH SEE ALSO
.sp
.LP
.BR mq_open (3C),
.BR mq_send (3C),
.BR mq_setattr (3C),
.BR mqueue.h (3HEAD),
.BR attributes (7),
.BR standards (7)
